A surface plasmon resonance (SPR) refers to a phenomenon of the propagation of surface plasmon polaritons (SPPs) which are formed on or near the surface of conductive materials by coupling of electrons and photons having a specific wavelength. In general, SPR is a collective oscillation of conduction band electrons propagating along the interface between a metal with a negative dielectric constant and a medium with a positive dielectric constant. SPR has an enhanced intensity in comparison with an incident electromagnetic wave and shows characteristics of an evanescent wave which exponentially decreases as getting far-off perpendicularly from the interface.
The SPR can be classified as propagating plasmons observed at the interface between a 10˜200 nm-thick flat metal surface and a dielectric substance; and a localized SPR (LSPR) observed from nanoparticles or nanostructures. LSPR detects a change in the SPR wavelength showing maximum absorbance or scattering which depends on a change of the chemical and physical environment on the surface (for example, a change in refractive index of a medium near the surface) of nanoparticles or nanostructures. A LSPR-based sensor has many merits over a conventional bulk SPR sensor which utilizes a plasmon propagation by prism coupling, because the detection of the SPR wavelength change permits to distinguish specific molecules or to analyze concentration of specific molecules in a medium; LSPR is highly sensitive to the change of refractive index and that allows label-free detection.
Nano-patterning technologies, such as an electron beam lithography, a focused ion beam, and a nano-imprint, have been proposed as methods for preparing metal nanoparticle array to induce LSPR. However, such conventional technologies have limitations in improving production yield upon applying continuous processes or various substrate sizes. Furthermore, the conventional technologies often lead to defective products due to defects or contaminations, in the case of requiring a mechanical contact. As another manufacturing method, a technology was proposed to deposit a metal thin film having a continuous profile and then thermally anneal to form an array of nano-islands isolated each other. In this case, however, substrate materials for nano-island formation are limited to heat-resisting materials such as glass and there is a problem of not obtaining a nanoparticle array with a high nanoparticle density.